1. Field of the Invention
This invention relates in general to semiconductor memories and, more specifically, to devices and methods for margin testing a semiconductor memory by, for example, equilibrating all complementary and true digit lines of the memory to ground simultaneously during a test mode.
2. State of the Art
As shown in FIG. 1, one conventional method for margin testing a sub-array 10 of a semiconductor memory begins with storing a supply voltage VCC on all memory cell capacitors 12 of the sub-array 10. This is accomplished by writing logical xe2x80x9c1xe2x80x9d bits to memory cells attached to true digit lines D0, D1, etc. using sense amplifiers 14 and even row lines R0, R2, etc., and by writing logical xe2x80x9c0xe2x80x9d bits to memory cells attached to complementary digit lines D0*, D1*, etc. using the sense amplifiers 14 and odd row lines R1, R3, etc. A cell plate voltage DVC2 equal to one-half the supply voltage VCC is applied to the cell plate of each memory cell capacitor 12.
Once each memory cell capacitor 12 has stored the supply voltage VCC, the row line R0, for example, is fired, causing the memory cells attached to the row line R0 to dump their stored charge from their memory cell capacitors 12 onto the true digit lines D0, D1, etc. This causes the sense amplifiers 14 to pull each of the true digit lines D0, D1, etc. up to the supply voltage VCC, and to pull each of the complementary digit lines D0*, D1 *, etc. down to ground. As a result, a full VCC-to-ground voltage drop is imposed across NMOS access devices 16 of the memory cells attached to the complementary digit lines D0*, D1*, etc. The VCC-to-ground voltage drop is maintained across the NMOS access devices 16 for a predetermined refresh interval of typically about 150 to 200 milliseconds (ms). This stresses any xe2x80x9cleakyxe2x80x9d NMOS access devices 16 and causes any such NMOS access devices 16 to lose significant charge from their memory cell capacitors 12 to the complementary digit lines D0*, D1*, etc. to which they are attached.
Once the predetermined refresh interval is over, all of the memory cells attached to the complementary digit lines D0*, D1*, etc. are read. Any of these cells that read out a logical xe2x80x9c1xe2x80x9d bit as a result of leaking excessive charge, instead of reading out the logical xe2x80x9c0xe2x80x9d bit they originally stored, are flagged as failing the margin test.
Once the memory cells attached to the complementary digit lines D0*, D1*, etc. have been tested, the memory cells attached to the true digit lines D0, D1, etc. are tested by firing the row line RI, for example. This causes the memory cells attached to the row line RI to dump their stored charge from their memory cell capacitors 12 onto the complementary digit lines D0*, D1*, etc. In turn, this causes the sense amplifiers 14 to pull each of the complementary digit lines D0*, D1 *, etc. up to the supply voltage VCC, and to pull each of the true digit lines D0, D1, etc. down to ground. As a result, a full VCC-to-ground voltage drop is imposed across NMOS access devices 18 of the memory cells attached to the true digit lines D0, D1, etc. The VCC-to-ground voltage drop is maintained across the NMOS access devices 18 for another predetermined refresh interval of about 150 to 200 ms. This stresses any xe2x80x9cleakyxe2x80x9d NMOS access devices 18 and causes any such NMOS access devices 18 to lose significant charge from their memory cell capacitors 12 to the true digit lines D0, D1, etc. to which they are attached.
Once the predetermined refresh interval is over, all of the memory cells attached to the true digit lines D0, D1, etc. are read. Any of these cells that read out a logical xe2x80x9c0xe2x80x9d bit as a result of leaking excessive charge, instead of reading out the logical xe2x80x9c1xe2x80x9d bit they originally stored, are flagged as failing the margin test.
This conventional margin testing method thus typically takes two predetermined refresh intervals of about 150 to 200 ms. each to complete. Since row lines in different sub-arrays in a semiconductor memory typically cannot be fired simultaneously because the addressing of the row lines is local to their respective sub-arrays, this conventional method cannot be used on more than one sub-array at a time. As a result, in a semiconductor memory containing four sub-arrays, for example, the conventional method described above takes approximately 1.2 to 1.6 seconds to complete. Because of the large number of semiconductor memories that typically require margin testing during production, it would be desirable to find a margin testing method that can be completed more quickly than the method described above.
As shown in FIG. 2, another conventional method for margin testing a semiconductor memory has been devised to conduct margin tests more quickly than the method described above. In this method, a sense amplifier 20 includes equilibrating NMOS transistors 22 which equilibrate true and complementary digit line pairs D0, D0*, etc. to a bias voltage VBIAS on an equilibrate bias node 23 in response to an equilibrate signal EQ. It should be understood that an xe2x80x9cequilibrate bias nodexe2x80x9d is a node to which a cell plate is coupled, and from which a bias voltage is globally distributed for use by equilibrating transistors throughout a semiconductor memory.
When the semiconductor memory is not in a margin test mode, a test mode signal TESTMODE is inactive, so that a PMOS transistor 24 is on and the bias voltage VBIAS on the equilibrate bias node 23 is equal to the cell plate voltage DVC2 on the cell plate 25. When the semiconductor memory is in a margin test mode, the test mode signal TESTMODE is active so that the PMOS transistor 24 isolates the equilibrate bias node 23 from the cell plate 25, and so an NMOS transistor 26 connects the equilibrate bias node 23 to a probe pad 28 positioned on the exterior of the semiconductor memory. A stressing voltage, such as ground, can then be applied to the probe pad 28 during margin testing to simultaneously stress memory cells attached to both the true and complementary digit lines D0, D0*, etc.
Because the method described immediately above does not require the firing of any row lines in order to stress memory cells, all cells in a semiconductor memory can be stressed at once using this method. As a result, this method only requires one predetermined refresh period to complete testing, no matter how many sub-arrays a semiconductor memory contains. Thus, the method dramatically improves the speed with which margin testing can be completed.
Unfortunately, the method described above with respect to FIG. 2 has proven difficult to implement because the PMOS transistor 24 generally does not reliably isolate the equilibrate bias node 23 from the cell plate 25. In addition, the probe pad 28 has proven to be a cumbersome addition to the exterior of a semiconductor memory.
Therefore, there is a need in the art for a device and method for margin testing a semiconductor memory that avoid the problems associated with the probe pad method described above while still providing a rapid margin testing device and method.
Circuitry, in accordance with the invention for margin testing a semiconductor memory, such as a Dynamic Random Access Memory (DRAM), includes switching circuits formed within the memory. Each switching circuit can be conveniently incorporated into a sense amplifier of the memory, and each is associated with a pair of digit lines of the memory to which it selectively applies a stressing voltage at substantially the same time during a margin test mode of the memory. The stressing voltage can be ground when all the memory cells of the memory store a supply voltage level during the margin test, or it can be at the supply voltage level when all the memory cells store a ground voltage level during the margin test. The switching circuits can apply the stressing voltage to the digit line pairs through equilibrating circuitry in the sense amplifiers into which they can be incorporated, or can apply the stressing voltage through other means. Also, isolating circuitry can be provided to isolate the digit line pairs during the margin test from an equilibrate bias node of the memory from which they normally receive a digit line equilibrating bias voltage, such as a cell plate voltage or the supply voltage.
The inventive margin testing circuitry thus stresses all memory cells in the memory at the same time without the need to fire individual row lines in different sub-arrays. As a result, it substantially reduces the time it takes to complete a margin test of the memory. Also, by positioning the switching circuits within the memory, the inventive margin testing circuitry avoids the cumbersome nature of the external probe pad of prior margin testing devices.
In other embodiments of the invention, a semiconductor memory, an electronic system, and a semiconductor substrate (e.g., a semiconductor wafer) incorporate the inventive margin testing circuitry described above.
In another embodiment of the inventionxe2x80x94a method of margin testing a DRAM a high voltage level is stored in memory cells of the DRAM. Equilibrating circuitry in sense amplifiers of the DRAM is isolated from an equilibrate bias node of the DRAM and from a cell plate voltage thereon, and a ground voltage from within the DRAM is applied to the equilibrating circuitry in each sense amplifier. Digit line pairs of the DRAM are then equilibrated to the ground voltage using the equilibrating circuitry in each sense amplifier, and the digit line pairs are held at the ground voltage for a predetermined refresh interval in order to stress the memory cells of the DRAM, which are attached to the digit line pairs. After the predetermined refresh interval has passed, all the memory cells of the DRAM are read to identify those that have failed the margin test. The refresh interval may be, for example, about 150 to 200 milliseconds.
In still another embodiment of the inventionxe2x80x94a method of testing a semiconductor memoryxe2x80x94a substantially identical logic voltage is stored in all memory cells of the semiconductor memory. Also, digit line pairs of the semiconductor memory that are attached to the memory cells are isolated from a digit line equilibrating bias voltage. This is accomplished by deactivating an NMOS transistor coupled between the bias voltage and the digit line pairs, or by failing to activate equilibrate circuitry coupled between the bias voltage and the digit line pairs that normally is activated during memory operations of the semiconductor memory. A stressing voltage from within the semiconductor memory that is substantially different than the logic voltage stored in the memory cells is then applied to all the digit lines of all the digit line pairs at substantially the same time, thereby stressing the memory cells. The digit line pairs are held at the stressing voltage for a predetermined interval, and all the memory cells of the semiconductor memory are then read to identify those that have failed the test.